1. Field of the Invention
The present invention relates to a filter with a selected attenuation/wavelength characteristic, and a method for making the same.
2. Related Art
It is known that the erbium (or other rare earth) doped fibre amplifiers or other types of fibre amplifiers are very attractive devices for optical communications and are likely to become widely used in place of optoelectronic repeaters. However, as it is generally desirable to multiplex signals by means of wavelength division multiplexing (WDM), the gain spectrum of an erbium doped fibre amplifier (simplified in FIG. 1) is not ideal having one large peak A and a (usually) lower peak B and a flatter spectrum over the desired range R, as shown in FIG. 2, would be desirable, especially where a series of amplifiers are used, to avoid large differences in gain across the wavelength band. Thus if the different signals which are to be multiplexed are provided at wavelengths .lambda..sub.1, .lambda..sub.2. . . .lambda..sub.5 as shown in FIG. 1, the signal at wavelength .lambda..sub.2 will be amplified considerably greater than the signals at wavelength .lambda..sub.1 and .lambda..sub.3 or .lambda..sub.5 and the signal at wavelength .lambda..sub.4 will be amplified to a slightly less extent than .lambda..sub.2. This clearly creates problems particularly if the WDM signal is passed through several erbium doped fibre amplifiers. One method of obtaining a flatter gain spectrum is to introduce a filter into the system with a selected attenuation/wavelength characteristic to compensate for the variation of the spectrum from a preferred flat spectrum.
It will be understood, in addition to the peaks A and B the erbium spectrum includes fine detail (not shown). It is possible to eliminate peak A by means of a filter such as a known interference filter (eg: grating filter) with an attenuation characteristic shown in FIG. 3 having a maximum attenuation at C at the interference wavelength and an attenuation peak D at lower wavelengths by matching peak D with peak A. Such an arrangement is described in an article entitled "D-Fibre Filter for Erbium Gain Spectrum Flattening" (Electronics Letters, 16 Jan. 1992 Vol 28, No 2, page 131-132). Such a filter can improve the flatness of the gain spectrum to within 0.5 dB over a 30 nm span. FIG. 4 shows the erbium gain spectrum after passing through the filter with the characteristic of FIG. 3. Nevertheless, peak B of the amplifier gain spectrum remains, and indeed this feature can be accentuated when the signal is passed through a succession of erbium doped fibre amplifiers with such filters.
It will be understood that a filter for use with an erbium or other rare earth doped fibre amplifier should not be a reflection type filter since this will create problems with the amplifier. A preferred type of filter is an interference filter, and we will describe a variety of filters including Bragg side-tap gratings.
In this specification the term "optical" is intended to refer to that part of the electromagnetic spectrum which is generally known as the visible region together with those parts of the infra-red and ultraviolet regions at each end of the visible region which are capable, for example, of being transmitted by dielectric optical waveguides such as optical fibres.